This invention relates to coincident current magnetic printing heads and in particular to magnetic printing heads having improved signal-to-noise ratios.
Coincident current magnetic printing heads generally depend for their operation on the simultaneous occurrence of electrical signal pulses through conductive signal lines disposed in the vicinity of one or more magnetic pole pieces. The magnetic pole pieces are generally placed in close proximity to a magnetic recording medium which moves relative to the printing head. The writing of information onto the recording medium requires the concurrent presence of two pulses or similar signals on the conductive signal lines. The magnetic pole pieces, signal lines, and current strengths are selected so that a current pulse on only one of the signal lines is of insufficient magnitude to write information onto the recording medium. This is referred to as the half-select condition. A full select condition occurs when both pulses are present simultaneously so that their effects are additive. Particularly in magnetic printing heads, there are a plurality of magnetic pole pieces which may be spaced at a resolution of one hundred dots per inch or more. The signal conductors associated with these linear arrays of pole pieces are organized into a word and digit structure. That is to say, adjacent sets of pole pieces are connected with a single word line conductor and with a plurality of digit line conductors. The writing of information onto the recording medium from any one of these adjacent poles pieces is accomplished by providing a half-select signal on the word line and a half-select signal on the desired digit line or digit lines. The positions associated with the pole pieces receiving only the half-select signal from the word lines receive no information; however, those positions associated with the magnetic pole pieces additionally receiving half-select signals through the digit lines, do receive the desired information.
Important in the selection of full select and half-select signal levels is the hysteresis curve for the magnetic material of the recording medium. This curve determines the ease of switching the magnetic flux in the medium and determines the signal levels employed. One of the drawbacks of the coincident current method is that increasing the signal strength to assure information transfer also increases the half-select signals so that there is no improvement in signal-to-noise ratio. Accordingly, the signal level cannot generally be increased without also increasing the noise level. Similar problems exist in the design of coincident current magnetic core memory systems.
While the present invention is applicable to any coincident currnt printing head including transverse heads, longitudinal heads, and perpendicular heads, the transverse, magnetic printing head disclosed in application Ser. No. 060,921, filed July 26, 1979, now abandoned is exemplary of the coincident current magnetic printing heads to which the present invention is directed. Accordingly, this aforementioned application is hereby incorporated herein by reference. Coincident current printing heads described in the prior art essentially comprise at least two printed circuit boards, each having an elongated gap therein across which the signal conductors pass. The printed circuit boards which typically comprise a flexible material such as Mylar.sup.R have their respective gaps aligned and are positioned so that the signal lines crossing the gaps are substantially parallel to and opposite corresponding signal lines on adjacent printed circuit boards. The magnetic pole pieces are provided by a magnetic comb-like structure. This structure typically comprises a material such as nickel-iron with teeth projecting along the long dimension thereof spaced to achieve approximately a resolution of 100 teeth per inch. The comb is disposed through the elongated gap in the printed circuit boards so that the comb teeth project between the parallel lines bridging the elongated gaps. An arched anodized aluminum support member is then employed to support the structure consisting of the printed circuit boards and the magnetic comb One of the printed circuit boards has disposed thereon the word signal lines and the other printed circuit board has disposed thereon the digit signal lines. An epoxy coating is then disposed over the arched surface and subsequently ground and polished to produce a smooth surface for contacting the recording medium. Such a structure may also include a third printed circuit board with conductive lines which serve only as a sacrificial layer during the grinding operation. Like other coincident current magnetic printing head structures, this configuration also suffers from the aforementioned signal-to-noise problem.